Magnetically tunable constant-width band-reject corrugated ferrite waveguide filter



March 19, 1963 M. CRANE ETAL 3,082,384

MAGNETICALLY TUNABLE CONSTANT-WIDTH BAND-REJEICT CORRUGATED FERRITE WAVEGUIDE FILTER Filed Sept. 15, 1961 FERRITE- FILLED WAVEGUIDE HAVING FERRITE-FILLED STUBS, ALL CONDUCTIVELY COATED- VARIABLE MAGNETIC BARE TAPERED FERRITE FOR COUPLING LARGER AIR-FILLED GUIDE TO SMALLER FERRITE- FILLED GUIDE.

INVENTORS, I M I LTO N C R A N E BY ONALD J- ACE ATTOR/Vf) fat 3,082,384 Patented Mar. 19, 1963 3,082,384 MAGNETECALLY TUNABLE CONSTANT-WIDTH BAND-REJECT CGRRUGATED FERRITE WAVE- SUEDE FILTER Milton Crane, Stanford, and Donald J. Grace, Menlo Park, Catifi, assignors to the United States of America as represented by the Secretary of the Army Filed Sept. 15, 196i, Ser. No. 138,535 3 @iaims. (Ci. 3353-73) This invention relates to corrugated waveguide filters and more particularly to tunable corrugated waveguide filters.

A corrugated waveguide filter is basically a stopband structure with the waveguide acting as a high-pass filter. A typical corrugated filter has a first passband that starts at the cut-off frequency of the waveguide, and a first stopband that starts at the high frequency end of the first passband. At the high frequency end of the first stopband a second passband begins. Thus the passand stopbands in a typical corrugated filter are repetitive.

In a tunable corrugated waveguide filter the ultimate objective is to obtain a wide tuning range without a decrease in stopband width. The corrugated waveguide filter of our invention has a wide tuning range, and the stopband width is not materially decreased.

Corrugaed waveguide filters may be tuned by any one of several different methods. For example, mechanical tuning apparatus can be used, or electrically biased non-linear ferroelectric material may be used for tuning. However, in accordance with this invention, a ferrite material under the influence of a magnetic field is incorporated into the filter structure for tuning purposes. Mechanical tuning is generally undesirable due to construction diificulties, especially when more than one filter section is used because tracking arrangements must be used to obtain proper tuning of a plurality of filter sections. The use of biased non-linear ferro-electric materials for tuning purposes is undesirable when microwave frequencies are involved because at present these materials are too lossy at these frequencies. The use of ferrite materials under the influence of a magnetic field overcomes the above mentioned and other difiiculties of the mechanical type and the ferro-electric type tuners.

In accordance with this invention the first stopband is made extremely wide by proper design procedures, and then, by incorporating ferrite into the corrugated structure, the stopbands can be moved so that the first passband can be controlled. Thus, the first stopband controls the frequency range of the passband. The stopband of the corrugated structure is controlled by varying a D.-C. magnetic field. This varies the magnetization of the incorporated ferrite, thereby, changing the propagation factor of the structure.

It is, therefore, an object of this invention to provide a tunable corrugated waveguide filter.

Another object of this invention is to provide a corrugate waveguide filter that incorporates ferrite in the filter structure.

A further object of this invention is to provide a tunable corrugated waveguide filter that has a wide tuning range without any material change in stopband width over the tuning range.

The above mentioned and other objects of the invention will become apparent from the following description and accompanying drawing which shows a preferred embodiment of our invention.

Referring to the figure, ferrite-filled corrugated waveguide 1 comprises a metallized rectangular slab of ferrite. The ferrite is machined to desired dimensions and tapered at its ends 2. The rectangular walls of the ferrite are metallized with any suitable metallic conductor such as copper-plated platinum. If copper-plated platinum is used, the platinum may be painted on the walls of the ferrite and then plated with copper in an acid bath using very low current. The structure thus far described is a ferrite waveguide. Stubs 4 are shaped separately out of pieces of ferrite; they form the slots of the air-dielectric corrugated structure. After machining, the stubs are metallized over their outer surfaces except where they attach to the ferrite waveguide. The stubs are attached to the Waveguide by means of slots ground into the ferrite waveguide at measured distances. been carefully placed in the slots, the entire structure is electroformed. The electroforming assures good electrical contact between the junction of the waveguide and the stubs. Tapered ends 2 of the ferrite waveguide are inserted into transitions 5. Transitions 5 are used to match the ferrite waveguide to any suitable rectangular guide (not shown).

Tuning of the corrugated filter is accomplished by subjecting the ferrite material to a D.-C. magnetic field. An'y suitable electromagnetic means may be used to obtain the magnetic field. In the figure only the core 3 of the magnetic structure is shown.

Stubs 4 are folded over so that only one magnetic field is used to properly magnetize the ferrite. As the magnetic field is increased the stopbands are moved up in frequency. Thus, the band width of the first passband increases. The bandwidth of the first stopband is essentially constant over the tuning range. It is only at the extreme high end of the tuning range that stopbands begin to deteriorate. This is extremely important because decrease in stopband width is a highly undesirable feature.

While the fundamental features of the invention as applied to a preferred embodiment have been shown and described in detail, it will be understood that various omissions, substitutions, and changes in form of the device illustrated, and in its operation, may be made by those skilled in the art, without departing from the spirit of the invention. For example, a typical air-dielectric corrugated waveguide filter having its slots partially or wholly filled with ferrite may be tuned in the manner taught by this invention. Therefore, it is the intention to be limited only as indicated by the scope of the following claims.

What is claimed is:

l. A tunable corrugated waveguide filter comprising: a ferrite waveguide tapered at each end; a plurality of L shaped ferrite stubs integral with said ferrite waveguide; and means adjacent to said stubs for applying a variable magnetic field to said ferrite waveguide.

2. A corrugated waveguide filter comprising: a rectangular ferrite slab tapered at each end; a conductive metal bonded between the tapered ends of said ferrite slab; a

plurality of slots cut into the metallized portion of said ferrite slab; a plurality of ferrite stubs each having one end placed in one of said slots and its other end folded over said ferrite slab; Waveguide transitions coupled to the tapered ends of said slab; and means magnetically coupled to said stubs for applying a variable magnetic field to said slab.

3. A corrugated waveguide filter comprising: a ferrite Waveguide; a plurality of ferrite stubs each bonded at one end to said ferrite waveguide and folded over said ferrite waveguide at the other end; and means magneticah ly coupled to said stubs for applying a variable magnetic field to said ferrite waveguide.

References Cited in the file of this patent UNITED STATES PATENTS 3,001,154 Reggia Sept. 19, 1961 3,013,229 De Grasse Dec. 12, 1961 3,046,503 Cohn July 24, 1962 After the stubs have 

1. A TUNABLE CORRUGATED WAVEGUIDE FILTER COMPRISING. A FERRITE WAVEGUIDE TAPERED AT EACH END; A PLURALITY OF L SHAPED FERRITE STUBS INTEGRAL WITH SAID FERRITE WAVEGUIDE; AND MEANS ADJACENT TO SAID STUBS FOR APPLYING A VARIABLE MAGNETIC FIELD TO SAID FERRITE WAVEGUIDE. 